Green’s Dictionary of Slang

abraham’s willing n.

a shilling (5p).

1859		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1909		J. Ware Passing Eng. of the Victorian Era 1/2: Abraham’s Willing (Rhyming). Shilling. Generally reduced to willing, e.g., ‘Lend us a willing.’.